easydiffusion/ui/media/js/themes.js

const themeField = document.getElementById("theme")
var DEFAULT_THEME = {}
var THEMES = [] // initialized in initTheme from data in css

function getThemeName(theme) {
    theme = theme.replace("theme-", "")
    theme = theme
        .split("-")
        .map((word) => word.charAt(0).toUpperCase() + word.slice(1))
        .join(" ")
    return theme
}
// init themefield
function initTheme() {
    Array.from(document.styleSheets)
        .filter((sheet) => sheet.href?.startsWith(window.location.origin))
        .flatMap((sheet) => Array.from(sheet.cssRules))
        .forEach((rule) => {
            var selector = rule.selectorText
            if (selector && selector.startsWith(".theme-") && !selector.includes(" ")) {
                if (DEFAULT_THEME) {
                    // re-add props that dont change (css needs this so they update correctly)
                    Array.from(DEFAULT_THEME.rule.style)
                        .filter((cssVariable) => !Array.from(rule.style).includes(cssVariable))
                        .forEach((cssVariable) => {
                            rule.style.setProperty(cssVariable, DEFAULT_THEME.rule.style.getPropertyValue(cssVariable))
                        })
                }
                var theme_key = selector.substring(1)
                THEMES.push({
                    key: theme_key,
                    name: getThemeName(theme_key),
                    rule: rule,
                })
            }
            if (selector && selector == ":root") {
                DEFAULT_THEME = {
                    key: "theme-default",
                    name: "Default",
                    rule: rule,
                }
            }
        })

    THEMES.forEach((theme) => {
        var new_option = document.createElement("option")
        new_option.setAttribute("value", theme.key)
        new_option.innerText = theme.name
        themeField.appendChild(new_option)
    })

    // setup the style transitions a second after app initializes, so initial style is instant
    setTimeout(() => {
        var body = document.querySelector("body")
        var style = document.createElement("style")
        style.innerHTML = "* { transition: background 0.5s, color 0.5s, background-color 0.5s; }"
        body.appendChild(style)
    }, 1000)
}
initTheme()

function themeFieldChanged() {
    var theme_key = themeField.value

    var body = document.querySelector("body")
    body.classList.remove(...THEMES.map((theme) => theme.key))
    body.classList.add(theme_key)

    //

    body.style = ""
    var theme = THEMES.find((t) => t.key == theme_key)
    let borderColor = undefined
    if (theme) {
        borderColor = theme.rule.style.getPropertyValue("--input-border-color").trim()
        if (!borderColor.startsWith("#")) {
            borderColor = theme.rule.style.getPropertyValue("--theme-color-fallback")
        }
    } else {
        borderColor = DEFAULT_THEME.rule.style.getPropertyValue("--theme-color-fallback")
    }
    document.querySelector('meta[name="theme-color"]').setAttribute("content", borderColor)
}

themeField.addEventListener("change", themeFieldChanged)